Nuclear Energy in Central Europe 2000

ADVANTAGES OF ULTRASONIC TIME OF FLIGHT

Predrag Dukić, Ines Dukić

IQA, d.o.o. B.Cerjakov 36, 8250 Brežice, Slovenia p.dukic@siol.net

ABSTRACT

In service inspection (ISI) of aged power plant parts has a delicate task to provideinformation on presence, position, orientation and size of defects in material. As far as resultsof ISI are just a starting point for elaborate procedure of structural integrity assessment, theaccuracy and reliability of these results are particularly important.

Time of Flight Diffraction (TOFD) technique is regarded as superior ultrasonic

technique capable to fulfil required goals [1]. In Slovenia TOFD is approved in nowadaysroutine inspection of components of conventional power plants [2-14].

As there were no practical use of TOFD in Slovenian NPP yet, some experience fromSlovenian conventional thermal power plants are presented. Results of TOFD inspections ofturbine rotor and turbine inlet casing are shown [11, 12].

satisfy needs for assessing integrity of components in conventional PP, instead of detailed 3-Dfinite element simulation. Example of application of R6 procedure in conventional PP isshown in this paper [14]. Although R6 results are relatively conservative, this relativelysimple to use procedure could meet requirements for assessing safety margins in domesticNuclear power plant too.

1 INTRODUCTION

Maintaining of aged power plants is very common problem in Slovenia, since manyplants are older than 30 years. Structural integrity assessment of components of these plantsusually do not follow known procedures, like R6, R5 or BS 7910 [16, 17], neither detailed3 – D finite element method analyses, but rely on experts opinion, based on experience andresults of simple NDT or material properties testing results.Predrag Dukić, Ines Dukić page 2 of 8ADVANTAGES OF ULTRASONIC TIME OF FLIGHT …

Such praxis could follow to wrong conclusions, and making decisions without welldefined acceptance criteria disables reproducibility of assessment.

Examples of inspection using TOFD on vital conventional Power Plants components

(rotor casing, and rotor shaft) and example of using R6 procedure in evaluating structuralintegrity has been shown.

Application of this techniques which advantages were also approved in NESC I projectwould probably improve safety and quality assurance in domestic Nuclear Power Plant.

2 INTRODUCTION TO TOFD ULTRASONIC TECHNIQUE

Technique is based on diffraction effect which occurs on every defect in the materialwhen it is »illuminated« by ultrasonic waves. Tip of the crack acts as a transmitter of newultrasonic wave, which radiates spherically from the tip. Two probes are scanning over defectcontaining sample. Transmitter probe T is emitting the ultrasonic waves, whilst receiver probeR collects all signals that appear in the area of beam coverage. In A scan view (classicaldisplay of ultrasonic signals), two reference signals are obtained: lateral signal (L) and back-wall echo (BWE), defining the area of interest on time axis. That means that signals of defectsare expected to be placed between L and BWE. The position of signal depends of its time offlight, i.e. of length of path it traveled.

Advanced equipment developed for routine performing of TOFD inspection allows

presentation of inspection results in B-scan display. Selected section of A-scan is digitizedand averaged, amplitude of the signal being presented as certain level on gray scale. Whendata is collected as the probes scan over the sample, the averaged A-scans are recorded atregular intervals along the scan. The data is then presented in 2 dimensions as B-scan display.X axis shows the length of the scan, while time axis oriented as y axis correlates with depth.

The difference between A-scan and B-scan display can be seen on Figure 1, showingclearly the advantages of B-scan regarding detection ability, resolution etc.

Figure 1 Diffraction signals of same defects in A-scan view and B-scan view

3 TOFD INSPECTION OF TURBINE CASING

Examination and testing of 28MW turbine inlet casing is described. It was designed as apressure vessel and has a thick-wall section that is more prone to thermal fatigue. It has 6steam outlets, leading to steam governor valves, every outlet surrounded by 8 bolt-holes. Dueto geometry of the component and stress-strain conditions, the most interesting is the areabetween the pairs of bolt-holes between outlets.

The idea was to collect the crack size data, by performing repeated inspection by theultrasonic TOFD method over a certain period, hoping that the reproducibility and accuracyof the method would be good enough to show the crack propagation. That would lead to anestimate of the flaw growth rate during service.

TOFD examination of the inlet casing upper plate was performed manually, due togeometry and space available on the object. Results were collected in two consecutive years:1997 and 98. Examination variables were the same.

In this paper, B scans for pass between bolt-holes 4 and 5 are shown on Figure 2, sincethey are typical for this casing. Second scan revealed presence of many new defects as well aspropagation of some known defect tips. Detailed analysis was performed, resulting innumerical data, showing location of the defects or defect groups.

4 TOFD INSPECTION OF TURBINE ROTOR

The journal of bored turbine rotor shaft was inspected, after the pulse-echo examinationrevealed presence of many defects with signals equivalent to 5mm FBH. In TOFDexamination, the surface was scanned manually with two 2 MHz shock wave probes, theirposition monitored by encoder. Special consideration was given to precise marking of everystart position of the probes, in order to obtain complete but not overlapping array of scannedareas on the journal Figure 3.

Figure 3 Scanning areas and directions on rotor shaft journal

The obtained scans formed a kind of map where individual defects were marked. Themap shoved that besides many single defects, most of them were aligned in axial direction.Repeated scanning along this path resulted in B-scan shown on Figure 4 showing large125mm long echo from a long planar crack, with through-wall extent of 24mm.

Failure assessment according to R6 gives the following conclusion: considering present

cracks, stress condition caused by normal exploitation parameters and measured materialproperties, result is hardly but stil in safe area.

Sensitivity analisys of load show that only 10% varying load cause crack.Sensitivitystudy of material property influence shows that in case of normal/required mechanicalproperties it would not come to the failure, even with load variation of more than 20%

Based on our own experiences in using TOFD in Slovenian conventional Power Plants,which are in accordance with conclusions made in NESC I project we can make nextconclusions:

1. TOFD is very helpful and useful ultrasonic technique, in inspection castings, forgines and welds. Advantages of technique are its accuracy, reliability, fastness, and its very good reproducibility.2. Nuclear Electric R6 Structural integrity assessment procedure is fast and relatively simple to use, conservative, and it is in accordance with British Standard PD 6493 (Now BS 7910), and European Commission Brite Euram SINTAP (Structural INTegrity Assessment Procedure) procedure, which are proposals to European Standard.

Based on our first two conclusions, and recommendations for TOFD given inconclusions of NESC I we suggest practical application of TOFD and R6 in domesticNuclear Power Plant too.